1. Field of the Invention
The present invention related generally to a traction control system for an automotive vehicle for detecting wheel slippage, such as wheel-spin and wheel-skid so as, to adjust driving torque to be applied to a vehicular wheels. More specifically, the invention relates to a traction control system which has a variable wheel slippage detecting threshold depending upon vehicle speed, so that sensitivity of wheel slippage can be adjusted depending upon the vehicle speed.
2. Description of the Background Art
In general, wheel slippage is reflected by a difference of an actual vehicle speed and an assumed vehicle speed which is based on a wheel speed. When the actual vehicle speed is lower than the assumed vehicle speed, it means the driven wheel causes wheel spin. On the other hand, when the actual vehicle speed is higher than the assumed vehicle speed, it means the vehicle is skidding. Wheel-spinning is caused by loss of road/tire traction. Therefore, in such case, traction control has to be performed in order to prevent the vehicular wheel from spinning. On the other hand, wheel-skidding occurs during application of abrupt braking and is caused by locking of the wheel. For example, such traction controls have been disclosed in U.S. Pat. No. 3,893,535, issued on July 8, 1975, to M. H. Burckhardt, et al. and in Japanese Patent First Publication (Tokkai Showa) No. 59-68537, published on Apr. 18, 1984. In both cases, a rotation speed of a driven wheel which is driven by an engine output is compared with a rotation speed of a non-driven wheel which rotates freely. The rotation speed of the non-driven wheel is treated as a parameter reflecting an actual vehicle speed.
The detection of the wheel slippage as disclosed in the aforementioned prior art is not applicable for anti-skid or traction control for a four-wheel drive wehicle. Namely, a four-wheel drive vehicle has four driven wheels and no non-driven wheel which is free from an engine output.
Furthermore, when traction control is performed in a substantially precise manner upon starting up the vehicle running on the slippy or muddy road, driving torque tends to be reduced at a level at which no wheel-spin occurs at any wheels. This results in lack of driving torque to make the vehicle impossible to start-up. On the other hand, in order to allow the vehicle to start-up on the slippy or muddy road, precision of the traction control has to be lowered. Lowering of precision will allow wheel-spin on the wheels. This degrades drivability when the vehicle is running at relatively high speed.